An instance variable has a name beginning with @, and its scope is confined to whatever object self refers to. Two different objects, even if they belong to the same class, are allowed to have different values for their instance variables. From outside the object, instance variables cannot be altered or even observed (i.e., ruby's instance variables are never public) except by whatever methods are explicitly provided by the programmer. As with globals, instance variables have the nil value until they are initialized.

Instance variables do not need to be declared. This indicates a flexible object structure; in fact, each instance variable is dynamically appended to an object when it is first assigned.

Notice above that i does not report a value for @bar until after the set_bar method is invoked.